A protest song is a song that is associated with a movement for social change and hence part of the broader category of topical songs (or songs connected to current events). It may be folk, classical, or commercial in genre.
Among social movements that have an associated body of songs are the abolition movement, women's suffrage, the labor movement, the human rights movement, civil rights, the anti-war movement and 1960s counterculture, the feminist movement, the sexual revolution, the gay rights movement, animal rights movement, vegetarianism and veganism, and environmentalism.
Protest songs are frequently situational, having been associated with a social movement through context. "Goodnight Irene", for example, acquired the aura of a protest song because it was written by Lead Belly, a black convict and social outcast, although on its face it is a love song. Or they may be abstract, expressing, in more general terms, opposition to injustice and support for peace, or free thought, but audiences usually know what is being referred to. Beethoven's "Ode to Joy", a song in support of universal brotherhood, is a song of this kind. It is a setting of a poem by Schiller celebrating the continuum of living beings (who are united in their capacity for feeling pain and pleasure and hence for empathy), to which Beethoven himself added the lines that all men are brothers. Songs which support the status quo do not qualify as protest songs.